The present invention relates generally to waveform databases and more specifically to generating a variable persistence waveform database for displaying repetitive signals on a display of a measurement test instrument.
Waveform databases are used in measurement test instruments, such as oscilloscopes, spectrum analyzers and the like, for accumulating digital data samples of an acquired signal under test, such as a digital communications signal. The waveform database is a two-dimensional array of counters where the rows represent magnitude values and the columns represent acquisition times. Acquisition circuitry in the measurement test instrument samples the magnitude of the incoming signal and generates digital data samples defining a waveform record. Each digital data sample includes a magnitude component representing a voltage level, power level or the like and a time component representing the relative time each digital data sample is acquired relative to other digital data samples. The waveform record length is a user defined parameter. The digital data samples of the waveform records are accumulated by the counters of the waveform database with the magnitude component of the sample defining a row location and the time component defining the column location. The accumulated counts of the digital data samples are transformed into intensity levels or color values based on the number count of each of the respective counters. A display device, such a cathode ray tube, liquid crystal display or the like displays the intensity levels or color values.
U.S. Pat. No. 5,343,405 to Kucera et al. describes various hardware systems for generating a waveform database used for automatic extraction of pulse parametric values from a digital serial communications signal forming an “eye” pattern. One described hardware device is a communications signal analyzer that samples the communications signal and generates waveform traces from acquired digital data samples. The digital data samples from each trace are accumulated in the waveform database. The digital data samples from the acquired waveform traces are continuously added to the waveform data database until a “STOP COMMAND” is issued. A “CLEAR COMMAND” is used to empty the contents of the waveform database and start accumulating a new set of waveform traces. Histogram and statistical analysis of the contents of the waveform database can be continuously performed as the waveform traces are accumulated into the waveform database. The contents of the waveform database is transformed to pixel coordinates of a display device, such as a cathode ray tube, liquid crystal display or the like and displayed as varying intensity levels or different colors based on the count levels of magnitude and time coordinates in the waveform database.
U.S. Pat. No. 6,151,010 to Miller et al. describes a digital oscilloscope having a variable persistence display. The oscilloscope acquisition circuitry samples the incoming signal in response to the arming of a trigger circuit and stores digital data samples of the signal in a segmented memory. A trigger signal terminates the writing of digital data samples into the segmented memory and freezes the contents of the memory. A large number of acquisitions may be stored simultaneously before any of the data is read out to an accumulation persistence map generator. The accumulated data in the segmented acquisition memory is provided to the accumulation persistence map generator that scales and time aligns the acquisition records. The scaled and time aligned samples from the acquisition records are accumulated in a three-dimensional persistence map where two dimensions of the map correspond to amplitude and time and the third dimension is population. Once the accumulated data from the segmented memory is accumulated in the three dimension persistence map, the next acquisition of digital data samples of the incoming signal is stored in the segmented memory and frozen. A persistence map decay ages the population statistics held in the three-dimensional persistence map. In response to a user selected decay time, the persistence map decay proportionally ages each of the populations at the amplitude and time locations to provide an exponential decay. Periodically, a percentage of the populations of each voltage and time location are decimated based on the user selected decay time. This proportional aging maintains the statistical integrity of the three-dimensional persistence map. The amplitude-time-population statistics in the three-dimensional persistence map are converted into pixel information for the oscilloscope display. The population statistics are assigned to 32 different shades of a waveform trace color defined for the input channel of the oscilloscope.
A drawback to existing waveform databases and persistence maps arises from the continuous accumulation of the underlying data in the waveform databases and persistence maps. Any changes in the signal input, such as signal drift, signal noise and the like, will be accumulated in the waveform database or persistence map. For example, a user may wish to observe changes in the output of a telecommunications circuit as the bias on a laser is varied. Changing the bias on the laser will change the extinction ratio of the signal that is displayed as an eye pattern on the oscilloscope. The changes in the signal input caused by the tuning of the laser will be accumulated in the waveform database and the persistence map irrespective of the population aging of the persistence map. As stated in the '010 patent, the proportional aging the persistence map maintains the statistical integrity of the map. The result is a closing of the eye pattern of the input signal as changes in the laser bias varies the output signal of the communications circuit. Any histogram or statistical measurement on the accumulated data in the waveform database or the persistence map will be flawed by the continuous accumulation of the underlying signal data. As a result, users need to continuously clear the waveform database or persistence map and acquire new signal data while adjusting the laser to observe the changes produced by the adjustment and statistically measure the results of the changes.
What is needed is a method of acquiring signal data of a repetitive signal, such as a telecommunications signal, and generating a time aged waveform database as compared to continuously accumulating waveform data and performing population aging of the persistence map. The time aged waveform database should provide variable persistence that continuously tracks changes of the input signal and continuously changing histogram and statistical data on the current state of the accumulating waveform database.